Glass articles exhibiting a gradient photochromic effect are well known. Canadian Pat. No. 739,404, for example, describes a method for producing a glass plate having a photochromic portion and a non-photochromic portion which comprises forming a glass plate from a glass composition which can be rendered photochromic by a suitable heat treatment, and thereafter subjecting the glass plate to a heat treatment in a partitioned lehr. Alternatively, the heat treatment may be carried out while the glass is partly encased in a refractory block.
The partitioned lehr and the refractory block both function to produce a temperature gradient across the glass. This gradient is such that one portion of the glass reaches a temperature sufficient to develop good photochromic properties while the other portion does not. The portion of the glass which has not been exposed to the higher temperatures may be either non-photochromic or photochromic to a lesser extent than the fully treated glass. In either case, the glass is referred to as a gradient photochromic glass.
German Patentschrift No. 2,125,232 suggests a related method for producing gradient photochromic glass wherein continuous sheet composed of potentially photochromic glass is drawn past a heating source which is directed only to a selected edge portion of the sheet. The heated edge portion then develops the desired photochromic properties, whereas the unheated regions do not.
Neither of the above methods is compatible with current methods for the production of photochromic ophthalmic lens blanks, so as to permit the efficient manufacture of gradient photochromic ophthalmic lenses. The method of the German patent is suitable only for a continuous sheet drawing process. The concept of the partitioned lehr disclosed in the Canadian patent cannot be economically adapted to presently used photochromic glass heat treating lehrs which are designed to handle many lens blanks simultaneously on a continuous basis. Similarly, the use of refractory shielding blocks into which glass must be packed with fibrous insulation is not practical where large numbers of glass articles must be handled.
An alternate method for producing a gradient photochromic glass article is disclosed in U.S. Pat. No. 3,419,370. That patent describes a process for producing a silver halide photochromic glass by the ion exchange treatment of a halogen-containing glass in a silver salt bath. It is noted in that patent that a gradient in photochromic behavior across a glass body can be attained by varying the time and/or temperature at which different portions of the glass body are exposed to the silver ion exchange medium.
Ion exchange processes for producing graded photochromic properties are also unsuited for the commercial production of gradient photochromic ophthalmic lenses. The use of silver ion exchange media such as molten salt baths is complex and expensive. Also, ion exchange processes must be carried out on a batch-by-batch basis, and thus are not readily adaptable to the continuous high-volume production of ophthalmic lenses.
It is the principal object of the present invention to provide methods and apparatus for producing gradient photochromic ophthalmic lenses which are both simple and inexpensive.
It is a further object of the invention to provide methods and apparatus for producing gradient photochromic ophthalmic lenses which are fully compatible with heat treating processes and equipment presently used for the production of photochromic ophthalmic ware.
Other objects and advantages of the invention will become apparent from the following detailed description thereof.